Electrodeposition methods of gallium and gallium alloy films and related photovoltaic structures

ABSTRACT

Photovoltaic devices and methods for preparing a p-type semiconductor generally include electroplating a layer of gallium or a gallium alloy onto a conductive layer by contacting the conductive layer with a plating bath free of complexing agents including a gallium salt, methane sulfonic acid or sodium sulfate and an organic additive comprising at least one nitrogen atom and/or at least one sulfur atom, and a solvent; adjusting a pH of the solution to be less than 2.6 or greater than 12.6. The photovoltaic device includes an impurity in the p-type semiconductor layer selected from the group consisting of arsenic, antimony, bismuth, and mixtures thereof. Various photovoltaic precursor layers for forming CIS, CGS and CIGS p-type semiconductor structures can be formed by electroplating the gallium or gallium alloys in this manner. Also disclosed are processes for forming a thermal interface of gallium or a gallium alloy.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims priority to U.S.patent application Ser. No. 12/874,496, filed Sep. 2, 2010, the contentsof which are incorporated by reference in its entirety.

BACKGROUND

This invention generally relates to electro deposition processes ofgallium and gallium alloy films for fabrication of thin filmphotovoltaic devices such as those containing copper, indium, gallium,and/or selenium and as thermal interface materials.

For photovoltaic applications, two layers of semiconductor materialhaving different characteristics are generally used in order to createan electrical field and a resultant electrical current. The first layeris typically an n-type semiconductor material and is generally thin soas to let light pass through to an underlying p-type semiconductor layermaterial layer, which is often referred to as the absorbing layer. Theabsorbing layer in combination with the n-type semiconductor materiallayer provides a suitable band gap to absorb photons from the lightsource and generate a high current and an improved voltage. For thep-type layer, thin films of a copper-indium-gallium-diselenidesemiconductor material (i.e., CuInGaSe₂ and variations thereof, alsoreferred to as CIGS) or copper indium diselenide (i.e., CuInSe₂, alsoreferred to as CIS) or copper gallium diselenide (i.e., CuGaSe₂, alsoreferred to as CGS) have generated significant interest over the yearsfor their use in photovoltaic devices.

By way of example, the p-type CIGS layer is typically combined with ann-type CdS layer to form a p-n heterojunction CdS/CIGS device. Zincoxide and doped zinc oxide may be added to improve transparency. Thedirect energy gap of CIGS results in a large optical absorptioncoefficient, which in turn permits the use of thin layers on the orderof 1-2 μm. By way of example, it has been reported that the absorbedlayer band gap was increased from 1.02 electron-volts (eV) for a CuInSe₂(CIS) semiconductor material to 1.1-1.2 eV by partial substitution ofthe indium with gallium, leading to a substantial increase inefficiency.

Formation of the CIGS structure is typically by vacuum deposition,chemical deposition or electrodeposition. The most common vacuum-basedprocess co-evaporates or co-sputters copper, gallium, and indium, thenanneals the resulting film with a selenium or sulfur containing vapor toform the final CIGS structure. An alternative is to directlyco-evaporate copper, gallium, indium and selenium onto a heatedsubstrate. A non-vacuum-based alternative process deposits nanoparticlesof the precursor materials on the substrate and then sinters them insitu. Electrodeposition is another low cost alternative to apply theCIGS layer. Although electrodeposition is an attractive option forformation of gallium thin films, especially for photovoltaicapplications such as CIGS, current processes are generally notcommercially practical. Gallium is generally considered a difficultmetal to deposit without excessive hydrogen generation on the cathodebecause the gallium equilibrium potential is relatively high. Hydrogengeneration on the cathode causes the deposition efficiency to be lessthan 100% because some of the deposition current gets used to formhydrogen gas rather than to form the gallium film on the substrate orcathode. Low cathodic deposition efficiency due to excessive hydrogengeneration results in poor process repeatability, partly due to the poorcathodic efficiency, and most importantly to poor deposit film qualitywith high surface roughness and poor deposit morphology.

Accordingly, there is a need in the art for improved electrodepositionprocesses for depositing gallium and gallium alloys as well as novelphotovoltaic devices containing the same with increased band gap toprovide increased photovoltaic current.

SUMMARY

The present invention is generally directed to methods of forming ap-type semiconductor layer for a photovoltaic device. In one aspect, themethod comprises electroplating a first layer onto a conductive surfaceof a substrate, wherein said first layer is selected from the groupconsisting of a copper layer and a copper-gallium layer; electroplatinga second layer onto said first layer, wherein said second layer isselected from the group consisting of an indium layer, a gallium layer,an indium-gallium layer, a copper-indium diselenide layer, and acopper-gallium-diselenide layer; and optionally electroplating a thirdlayer onto said second layer, wherein said third layer is selected fromthe group consisting of a gallium layer and an indium layer; andoptionally electroplating a fourth layer onto said third layer, whereinsaid fourth is selected from the group consisting of selenium andsulfur; wherein said electroplating is carried out by a methodcomprising: contacting: (i) a substrate and (ii) a solution comprising:a precursor comprising an element selected from the group consisting ofcopper, gallium, indium, selenium, sulfur and a combination thereof;optionally a metalloid compound additive; further optionally an organicadditive having at least a sulfur atom; and a solvent to dissolve saidprecursors; wherein the solution is free of complexing agents; adjustinga pH of said solution to a range selected from the group consisting of apH of about zero to less than about 2.6 and a pH of about 12.6 to about14, and applying a current to electroplate said substrate to producesaid first, second, third or fourth layers; and annealing said first,said second and said third layers in the presence of a selenium sourceand/or sulfur source to form the p-type semiconductor layer.

In a method for forming a thermal interface, the method compriseselectroplating a layer of gallium or a gallium alloy onto a heatemitting surface coupled to a microprocessor, wherein electroplating thegallium or gallium alloy comprises contacting the heat emitting surfacewith a plating bath free of complexing agents comprising a gallium saltand an optional organic additive comprising at least one sulfur atom,and a solvent; adjusting a pH of the plating bath to a range selectedfrom the group of a pH of greater than about zero to less than 2.6 and apH greater than about 12.6 to about 14, and applying a current toelectroplate the heat emitting surface to produce a layer of the galliumor gallium alloy; and coupling a heat sink or a heat spreader to thelayer of gallium or the gallium alloy to form the thermal interface

A photovoltaic device comprises at least one layer comprising gallium orindium or alloys comprising gallium and indium, wherein the at least onelayer is formed by electrodeposition; and an impurity in the at leastone layer selected from the group consisting of arsenic, antimony,bismuth, selenium, sulfur and mixtures thereof.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter that is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 depicts a cross sectional view of a CIGS precursor structure inaccordance with the present invention;

FIG. 2 depicts a cross sectional view of a CIGS precursor structure inaccordance with the present invention;

FIG. 3 depicts a cross sectional view of a CIGS precursor structure inaccordance with the present invention;

FIG. 4 depicts a cross sectional view of a CIGS precursor structure inaccordance with the present invention;

FIG. 5 depicts a cross sectional view of a gallium thermal interface;

FIG. 6 schematically illustrates an exemplary electrodepositionapparatus for deposition of a gallium layer onto a substrate;

FIG. 7 shows a scanning electron micrograph of a cross-sectional view ofa film stack wherein gallium was electrodeposited onto an indium layerand subsequently annealed to form an indium rich gallium eutectic layer;

FIGS. 8 and 9 show scanning electron micrographs of top down views ofgallium galvanostatically deposited from an acidic methane sulfonic acidsolution with added thiourea at 20 and 30 mAcm-², respectively; and

FIG. 10 graphically illustrates cyclic voltammetry plots for acidicgallium plating baths with no additives, with arsenic trioxide additive,and with arsenic trioxide and thiourea as additives.

The detailed description explains the preferred embodiments of theinvention, together with advantages and features, by way of example withreference to the drawings.

DETAILED DESCRIPTION

The present invention provides low cost electrodeposition processes forforming thin layers of gallium and/or gallium alloys such as may bedesired for forming various photovoltaic devices (e.g., CIGS, CIS, CGS,and the like) and as a thermal interface.

The electrodeposition processes utilize electroplating solutions todeposit compositionally pure, uniform, substantially defect free, andsmooth thin films with high plating efficiency and repeatability. Theelectroplating solutions are free of complexing agents and can bepracticed at both high and low pH ranges. Thin films of alloys may alsobe electroplated. Examples of desirable gallium alloys generally dependon the intended application and can include, without limitation, binary,ternary or higher order alloys of silver, copper, indium, zinc, tin,lead, silver, bismuth, gold, selenium, sulfur, and the like. Optionally,the alloy can be formed by annealing a film stack including anelectrodeposited gallium layer and one or more alloying element metallayers. In this manner, low cost fabrication of gallium or gallium alloythin films is achieved wherein the gallium layer or the gallium alloylayer is of uniform thickness, excellent morphology, and substantiallydefect free.

In one embodiment, gallium is electrodeposited to form a CIGS precursorstructure configured to control gallium inter-diffusion within the filmstack. In the exemplary embodiment shown in FIG. 1, a conductive layer14 is first deposited onto the substrate 12, which serves as a metalback contact. The conductive layer may include, without limitation,molybdenum, tantalum, tungsten, titanium, the corresponding nitridesthereof, and the like. The conductive layer is generally deposited byany means at a thickness of about 300 nm to about 600 nanometers (nm). Acopper layer 16 is then disposed onto the conductive layer 14 at athickness of about 10 nm to about 500 nm; in other embodiments, thecopper layer is at a thickness of 220 nm to 260 nm; and in still otherembodiments, the copper layer is at a thickness of about 240 nm. Anindium layer 18 is then deposited onto the copper layer 16 at athickness of 50 nm to 500 nm; in other embodiments, the indium layer isat a thickness of 375 nm to 425 nm; and in still other embodiments, theindium layer is at a thickness of about 420 nm. The gallium layer 20 isthen deposited onto the indium layer 18 at a thickness of 20 nm to 200nm; in other embodiments, the gallium layer is at a thickness of 100 nmto 150 nm; and in still other embodiments, the gallium layer is at athickness of about 140 nm. The gallium layer is deposited using theelectrodeposition process in accordance with the present disclosure. Theother layers may be deposited by any deposition technique, e.g. vacuumdeposition, but it is generally preferred that these layers be depositedby electrodeposition.

FIG. 2 illustrates an exemplary film stack 30 suitable as a CIGSprecursor structure in accordance with another embodiment of the presentdisclosure. In this exemplary embodiment, a conductive layer 34 is firstdeposited onto the substrate 32 at a thickness of about 300 nm to about600 nm. A copper-gallium alloy layer 36 is then electrodeposited ontothe conductive layer 34 at a thickness of 275 to 330 nm, e.g., 310 nm.An indium-gallium layer 38 is then electrodeposited onto thecopper-gallium layer 36 at a thickness of 420 to 500 nm, e.g., 490 nm.The ratio of Cu/(In+Ga) can be maintained at 0.8 to 0.9 e.g., 0.88 andthe ratio of Ga/(Ga+In) is can be maintained at 0.3 to 0.33, e.g., 0.31.It should be apparent in view of this disclosure that the conductivelayer may be deposited by any deposition technique but it is generallypreferred that these layers be deposited by electrodeposition. Galliumis a very low melting point element. It is liquid at about 35° C. and asa result is very mobile and inter-diffuses readily. Alloying galliumwith higher melting point metals such as copper and indium not onlyreduces the number of electrodeposition process steps but alsostabilizes the microstructure and allows better inter-mixing of theprecursor CIGS material. This ultimately results in better compositionalcontrol of the CIGS p-absorber material.

FIG. 3 illustrates an exemplary film stack 50 suitable as a CIGSprecursor structure in accordance with another embodiment of the presentdisclosure. In this exemplary embodiment, a conductive layer 54 is firstdeposited onto the substrate 54. A copper layer 56 is then disposed ontothe molybdenum layer 56. The thicknesses of the copper and molybdenumlayers are as previously described. An indium-gallium layer 58 is thenelectrodeposited onto the copper layer 56 at a thickness of 400 nm to500 nm and most precisely 490 nm. The ratio of Cu/(In+Ga) can bemaintained at 0.8 to 0.9, e.g., 0.88 and the ratio of Ga/(Ga+In) can bemaintained at 0.3 to 0.33, e.g., 0.31. The conductive and copper layersmay be deposited by any deposition technique but it is generallypreferred that these layers be deposited by electrodeposition.Deposition of the InGa alloy provides better control of CIGS precursorintermixing and final control of the CIGS composition.

FIG. 4 illustrates an exemplary film stack 60 suitable as a CIGSprecursor structure in accordance with another embodiment of the presentdisclosure. In this exemplary embodiment, a conductive layer 64 is firstdeposited onto the substrate 62. A copper-indium-selenium layer 66 isthen electrodeposited onto the molybdenum layer 64 at a thickness of 1micron to 2.5 microns. A gallium alloy layer 68 is then electrodepositedonto the copper-selenium-indium layers 66. The thicknesses of themolybdenum and gallium layers are as previously described. It should beapparent in view of this disclosure that the conductive layer may bedeposited by any deposition technique but it is generally preferred thatthese layers be deposited by electrodeposition. With this method, someof the indium in the CuInSe2 material is substituted by gallium formingCuInGaSe₂ upon annealing.

The films stacks including the copper, gallium, and indium layers asdescribed above in relation to FIGS. 1-4 are then reacted with seleniumand/or sulfur to form a CuInGaSe₂ or CuInGaSe₂S or CuInGaS structure.For example, a selenium layer may be deposited onto the film stack andsubsequently annealed to form the selenide. Alternatively, the filmstack can be exposed to hydrogen selenide and/or hydrogen sulfide, forexample, and subsequently annealed. Annealing in sulfur and/or seleniumatmosphere may occur at a temperature of about 400° C. to about 700° C.and preferably 550° C. It should be apparent to those skilled in the artthat subsequent to CIGS formation, deposition of an n-type junctionlayer (not shown) is then disposed onto the CIGS layer. As noted above,this layer will interact with the CIGS layer to form a p-n junction. Thenext layer to be deposited is typically a ZnO and doped ZnO transparentoxide layer (not shown). Moreover, it should be apparent that theelectroplating process can be utilized to form precursor layers forother types of photovoltaic devices, e.g., copper-indium-selenium (CIS),copper-gallium-selenium (CGS), copper-indium-sulfur (CISu), coppergallium sulfur (CGSu) and the like.

In the various embodiments described above, the resulting CIGSstructures generally have a Cu/(in+Ga) ratio of 0.8 to about 0.9 and aGa/(Ga+In) ratio of 0.3 to about 0.33.

In another embodiment, a gallium layer or a gallium alloy layer iselectrodeposited to form a thermal interface. The layer of gallium orgallium alloy can be electroplated as a stack on the underlayer of Zn,Sn, In, Au, Cu, mixtures thereof, of the like. Gallium provides lowtensile strength as well as high bulk thermal conductivity. As an alloy,self diffusion provides a low melting alloy suitable for its applicationas a thermal interface material. As such, gallium can be alloyed withother elements to lower the melting. FIG. 5 illustrates an exemplarydevice including a microprocessor chip coupled to a heat sink to preventoverheating by adsorbing its heat and dissipating the heat into the air.The device 100 includes a substrate 102 upon which the microprocessor104 is formed and mounted. A gallium or gallium alloy layer 106 iselectrodeposited onto a surface of the microprocessor. A heat sink 108is then coupled to the gallium layer. Table 1 provides exemplary galliumalloys suitable for use as a thermal interface and the correspondingliquidus and solidus temperatures.

TABLE 1 Composition Liquidus (° C.) Solidus (° C.)61.0Ga/0.25In/13.0Sn/1.0Zn 7.6 6.5 62.5Ga/21.5In/16.0Sn 10.7 10.775.5Ga/24.5In 15.7 15.7 95Ga/5In 25.0 15.7 100Ga 29.8 29.8

The electrodeposition processes for forming the gallium or gallium alloylayers generally include electroplating a substrate surface (e.g., aworking electrode) disposed in an aqueous plating bath comprising agallium salt, a methane sulfonic acid (MSA) electrolyte, and a solvent.The pH of the bath can be controlled using an acid or a base. Theconcentration of gallium ions in the electrolyte may range from about0.000005 Molar (M) M up to molar concentrations close to the saturationlimit in the electrolyte and pH used. Useful gallium sources for theplating bath include gallium salts soluble within the plating bathincluding, without limitation, gallium chloride (GaCl₃), gallium bromide(GaBr₃), gallium iodide (Gal₃), gallium nitrate Ga(NO₃)₃, galliumsulfate Ga(SO₄)₃, mixtures thereof, and the like. Other suitable galliumsalts include salts of sulfuric acid, sulfamic acid, alkane sulfonicacid, aromatic sulfonic acid, fluoroborate, and strong bases such assodium hydroxide, potassium hydroxide, lithium hydroxide, calciumhydroxide, magnesium hydroxide, and the like.

The concentration of acid such as MSA as the electrolyte may range fromabout 0.1 M to about 2 M; in other embodiments, the acid is in a rangeof about 0.1 M to 1 M; and in still other embodiments, the acid is in arange of 0.5 M to 1 M. As described, the electrolyte bath is free fromany kind of organic or inorganic complexing agents. That is, the galliumsalt is soluble within the electrolyte bath.

The pH of the electrolyte bath is generally less than 2.6 or greaterthan 12.6. Applicants have discovered that the plating bath becomescloudy, i.e., milky like in appearance, when the solution pH is in therange of 2.6≦pH≦12.6. While not wanting to be bound by theory, it isbelieved that oxides and/or hydroxides of gallium are formed within thispH range, e.g. gallium oxides and hydroxides in aqueous solutions.Suitable acids or bases to provide and maintain the pH of theelectrolyte bath are exclusive of complexing agents and may include,without limitation, mineral acids such as sulfuric acid, organic acidssuch as methane sulfonic acid, ethane sulfonic acid, propane sulfonicacid, butane sulfonic acid or other alkane sulfonic acid and aromaticsulfonic acid such as benzene sulfonic acid, and toluene sulfonic acid.Advantageously, it has been discovered that the electrodepositionprocesses at these pH ranges provide a uniform, thin conformal galliumlayer, thereby preventing individual island formation.

The alloying elements may be added directly to the bath. For example,copper in the electrolyte may be provided by a copper source such asdissolved copper metal or a copper salt such as copper sulfate, copperchloride, copper acetate, cupper nitrate, and the like. Likewise, indiummay be provided in the electrolyte by an indium source such as indiumchloride, indium sulfate, indium sulfamate, indium acetate, indiumcarbonate, indium sulfate, indium phosphate, indium oxide, indiumperchlorate, indium hydroxide, and the like.

The gallium electroplating bath may further include an optional organicadditive comprising at least one nitrogen atom or at least one sulfuratom. The organic additive is added to the plating bath to effectivelyincrease hydrogen evolution over-potential and prevent or effectivelylimit the co-deposition/evolution of hydrogen during plating of galliumand to control microstructure of the deposit by controlling nucleationand growth. Advantageously, the additive also functions as a brightenerand grain refiner while concomitantly assisting with gallium nucleation.Thinnest layers are formed by instantaneous nucleation where the samesize islands form simultaneously on a surface. Also, thin layers can beformed by progressive nucleation where formation of islands is afunction of time. In doing so, the resulting gallium layer is uniformand conformal, thereby preventing large three dimensional islandformations during deposition. Exemplary organic additives include,without limitation, aliphatic and/or heterocyclic compounds such asthioureas, thiazines, sulfonic acids, sulfonic acids, allyl phenylsulfone, sulfamides, imidazoles, amines, isonitriles,dithioxo-bishydroxylaminomolybdenum complex, and derivatives thereof.

The organic additive comprising the at least one nitrogen atom and/or atleast one sulfur atom additive has been found to unexpectedly accelerategallium plating while suppressing hydrogen evolution. In this manner, ithas been discovered that the organic additive provides a synergisticeffect when employed in combination with MSA as the electrolyte. Theconcentration of the organic additive comprising the at least onenitrogen atom and/or at least one sulfur atom may range from about 1parts per million (ppm) to about 10000 ppm, in other embodiments, theorganic additive is in a range of about 10 ppm to 5000 ppm, and in stillother embodiments, the organic additive is in a range of 100 ppm to 1000ppm.

In other embodiments, a metal oxide is added in combination with theorganic additive to poison the cathode, thereby increasing the onsetover-potential of hydrogen evolution (i.e., inhibit hydrogen generation)and accelerating gallium deposition. The inorganic metal oxide includes,without limitation, oxides of metalloids such as arsenic oxides (e.g.,As₂O₃; As₂O₅, KH₂AsO₄, K₂HAsO₄, K₃AsO₄, K₃AsO₃, KAsO₂, NaH₂AsO₄,Na₂HAsO₄, Na₃ASO₄, Na₃ASO₃, NaAsO₂, Na₄AS₂O₇, and the like); antimonyoxides, (e.g., Sb₂O₃, Sb₂O₅, KH₂SbO₄, K₂HSbO₄, K₃SbO₄, K₃SbO₃, KSbO₂,NaH₂SbO₄, Na₂HSbO₄, Na₃SbO₄, Na₃SbO₃, NaSbO₂, Na₄Sb₂O₇, and the like);and bismuth oxides (e.g., Bi₂O₃, K₃BiO₃, KBiO₂, Na₃BiO₃, NaBiO₂ and thelike).

Gallium deposition and hydrogen evolution are known to occursimultaneously, and thus, prior art plating processes generally exhibitlow plating efficiencies in order to prevent hydrogen evolution, whichcontributes to porosity within the deposited film structure The metaloxides described above are effective cathodic poisons and advantageouslyincrease the onset of over-potential of hydrogen evolution andunexpectedly accelerate gallium deposition. Plating efficiencies greaterthan 90 to 95% have been observed with gallium plating solutionsincluding the combination of the metal oxide and the organic additivecomprising at least one nitrogen atom and at least one sulfur atom. Theconcentration of metal oxide in the electrolyte may range from about 1parts per million (ppm) to about 10,000 ppm, in other embodiments, themetal oxide is in a range of about 100 ppm to 5,000 ppm, and in stillother embodiments, the metal oxide is in a range of 1,000 ppm to 3,000ppm. By introducing the metal oxide and/or sulfur in the plating bath,the resulting layer will include the corresponding metal (e.g., arsenic,antimony, bismuth or mixtures thereof), and/or sulfur as an impurity onthe order of a few parts per million up to a few atomic percent in thedeposit, which can be detected using an Auger or SIMS analytical method.

In another embodiment, the plating bath includes a gallium salt, asodium sulfate (Na₂SO₄) electrolyte, an organic additive comprising theat least one nitrogen atom and/or at least one sulfur atom, and asolvent. The concentrations of the gallium salt and the organic additiveare as previously described. The concentration of sodium sulfate as theelectrolyte may range from about 0.01 M to about 2 M; in otherembodiments, the sodium sulfate is in a range of about 0.1 M to 1 M; andin still other embodiments, the sodium sulfate is in a range of 0.2 M to60 M. Optionally, the metal oxide as described above may be included inplating bath. The pH is less than 2.6 or greater than 12.6 as previouslydescribed.

In the various embodiments described above, the electroplating chemistrycan be used on conductive and non-conductive substrates. Suitableconductive substrates include, without limitation, gold, molybdenum,indium copper, selenium, zinc, and the like. Suitable non-conductivesubstrates generally are those having a metal seed layer thereon andinclude, without limitation, glass, quartz, plastic, polymers, and thelike. For example, the non-conductive substrate may include a seedlayer, e.g., a copper seed layer. The particular method for depositingthe seed layer is not limited and is well within the skill of those inthe art. For example, the seed layer may be formed by chemical vapordeposition, plasma vapor deposition, or electroless deposition.

The electroplating baths may also comprise additional ingredients. Theseinclude, but are not limited to, grain refiners, surfactants, dopants,other metallic or non-metallic elements etc. For example, other types oforganic additives such as surfactants, suppressors, levelers,accelerators and the like may be included in the formulation to refineits grain structure and surface roughness. Organic additives include butare not limited to polyalkylene glycol type polymers, polyalkanesulfonic acids, coumarin, saccharin, furfural, acryonitrile, magentadye, glue, starch, dextrose, and the like.

Although water is the preferred solvent in the formulation of theplating baths, it should be appreciated that organic solvents may alsobe added in the formulation, partially or wholly replacing the water.Such organic solvents include but are not limited to alcohols,acetonitrile, propylene carbonate, formamide, dimethyl sulfoxide,glycerin, and the like.

Although DC voltage/current can be utilized during the electrodepositionprocesses, it should be noted that pulsed or other variablevoltage/current sources may also be used to obtain high platingefficiencies and high quality deposits. The temperature of theelectroplating baths may be in the range of 5 to 90° C. depending uponthe nature of the solvent. The preferred bath temperature for waterbased formulations is in the range of 10 to 30° C.

Referring now to FIG. 6, in practice, a backside electrical contact 5 ismade to a conductive substrate 4, which functions as the workingelectrode, upon which gallium or gallium alloy is to beelectrodeposited. Alternatively, if the substrate is non-conducting, aconductive layer and/or a seed layer (not shown) can first be depositedand electrical contact can be made directly to the seed layer via ohmiccontact or to the underlying conductive layer. An electrolyte solution 1in accordance with the present disclosure is placed in contact with thesubstrate surface 4. A conductive counter electrode 6, i.e., anode orconductor, is positioned in the electrolyte solution and spaced apartfrom the substrate (working electrode). While the substrate 4 is shownas having a planar surface, it is understood that substrate 4 can alsohave some topography and/or conformal conductive layers thereon. Forelectrochemical processing, an electrical current or voltage is appliedto the substrate (electrode) 4 and the counter electrode 6 via a powersupply 7 and electrical leads 8. If desired, the electrochemicalpotential of the structure/electrolyte can be controlled more accuratelyby the introduction of a third electrode, that is, a reference electrode(not shown), which has constant electrochemical potential. Examples ofreference electrodes include a saturated calomel electrode (SCE) andsilver-silver chloride (Ag/AgCl) reference electrodes or other metalreference electrodes such as Cu or Pt. The electrolyte solution can beagitated during electrodeposition.

The following examples are presented for illustrative purposes only, andare not intended to limit the scope of the invention.

Example 1

In this example, gallium was electroplated onto a film stack andsubsequently self-annealed to form an indium rich indium-gallium alloy.The plating chemistry included 0.2M Ga³⁺ in 0.5M MSA quenched with 0.5 MNaOH and then adjusted to a pH of 1.21 using additional amounts of MSA.Gallium was electroplated onto a 360 nm indium layer and a 250 nm copperlayer. The gallium layer with a thickness of 150 nm was subsequentlyself-annealed at room temperature 18-22° C. for a period of 3 days. Uponplating gallium on indium, interdiffusion has onset immediately andprogressively formed In—Ga eutectic alloy.

FIG. 7 shows a scanning electron micrograph of a cross-sectional view ofa film stack wherein gallium was electrodeposited onto the indium layerand subsequently annealed to form an indium-rich gallium eutectic layer.Interestingly, the Ga interdiffusion did not stop at the indium layerand continued into the copper forming an alloy of CuInGa.

Example 2

In this example, various gallium plating baths with and without theorganic additive were used to electro deposit gallium onto glasssubstrates having thereon a molybdenum layer that had previously beenseeded with copper. The plating solution included 0.25 M gallium sulfatein 0.5 M MSA with 0 and 500 ppm of thiourea. The electrolyte bath was at18-20° C. and agitated at 0 and 550 rpm. The pH was maintained at 1.14using H₂SO₄.

The results show that the presence of the organic additive clearlyaccelerated gallium plating relative to plating baths that did notcontain the organic additive. Moreover, continuous agitation of theelectrolyte provided significantly higher current densities thanwithout. FIGS. 8 and 9 pictorially illustrate surface topographic viewsof the galvanostatically deposited gallium film at 20 mA/cm² and 30mA/cm², respectively. An increase in grain size was observed with theincreased current density. No porosity was observed and the films wereuniform and of excellent morphology.

Example 3

In this example, the plating bath included 0.2MGa³⁺ in 0.5M MSA quenchedwith 0.5M NaOH and then adjusted by adding more MSA to obtain a pH of1.18. Varying amounts of As₂O₃ were included in the plating bath, whereindicated. For the plating bath that included no As₂O₃ or thiourea, theplating bath included Ga³′ in 0.5 M MSA quenched with 0.5 M NaOH withthe pH adjusted to 1.18 using additional MSA. The plating bath thatincluded a combination of As₂O₃ and thiourea contained As₂O₃ was at500-6000 ppm and the thiourea was at 100-1000 ppm.

FIG. 10 provides an overlay of the various voltammetry plots andincludes data for the combination of As₂O₃ and thiourea. As shown, theincreasing amounts of As₂O₃ provided a negative potential shift foronset of hydrogen evolution over-potential, thereby effectivelyinhibiting hydrogen generation. In addition, the combination of thioureaand As₂O₃ accelerated gallium deposition.

Example 4

In this example, the plating bath included 0.25M Ga³⁺ in 0.5 M MSAquenched with 0.5 M NaOH and adjusted to a pH of 1.18 using additionalamounts of MSA. Plating was carried out without any additionaladditives, with 6000 ppm As₂O₃, and with 6000 ppm As₂O₃ and 500 ppmthiourea. Cyclic voltammetry plots of these plating chemistries areprovided in FIG. 10. Inhibition of hydrogen evolution and accelerationof gallium deposition was observed upon addition of As₂O₃ and furtherincreases in acceleration with the combination of As₂O₃ and thiourea. Ithas also been shown that the combination of As₂O₃ and As₂O₅ is alsoeffective for inhibiting the hydrogen evolution (results not shownhere). When both of these oxides are combined together then the effectis much effective even at lower concentrations.

All ranges disclosed herein are inclusive of the endpoints, and theendpoints are combinable with each other.

All cited patents, patent applications, and other references areincorporated herein by reference in their entirety.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. Further, it should further be noted that the terms “first,”“second,” and the like herein do not denote any order, quantity, orimportance, but rather are used to distinguish one element from another.

While the preferred embodiment to the invention has been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A method of forming a p-type semiconductor layer for a photovoltaicdevice, comprising: electroplating a first layer onto a conductivesurface of a substrate, wherein said first layer is selected from thegroup consisting of a copper layer and a copper-gallium layer;electroplating a second layer onto said first layer, wherein said secondlayer is selected from the group consisting of an indium layer, agallium layer, an indium-gallium layer, a copper-indium diselenidelayer, and a copper-gallium-diselenide layer; and optionallyelectroplating a third layer onto said second layer, wherein said thirdlayer is selected from the group consisting of a gallium layer and anindium layer; and optionally electroplating a fourth layer onto saidthird layer, wherein said fourth is selected from the group consistingof selenium and sulfur; wherein said electroplating is carried out by amethod comprising: contacting: (i) a substrate and (ii) a solutioncomprising: a precursor comprising an element selected from the groupconsisting of copper, gallium, indium, selenium, sulfur and acombination thereof; optionally a metalloid compound additive; furtheroptionally an organic additive having at least a sulfur atom; and asolvent to dissolve said precursors; wherein the solution is free ofcomplexing agents; adjusting a pH of said solution to a range selectedfrom the group consisting of a pH of about zero to less than about 2.6and a pH of about 12.6 to about 14, and applying a current toelectroplate said substrate to produce said first, second, third orfourth layers; and annealing said first, said second and said thirdlayers in the presence of a selenium source and/or sulfur source to formthe p-type semiconductor layer.
 2. The method of claim 1, wherein theconductive surface is selected from the group consisting of molybdenum,tantalum, tungsten, titanium, and corresponding nitrides thereof.
 3. Themethod of claim 1, wherein the plating bath comprises sodium sulfate ata concentration of 0.01 M to 2 M.
 4. The method of claim 1, wherein theplating bath further comprises an oxide of a metalloid.
 5. The method ofclaim 4, wherein the oxide of the metalloid is in an amount of 1 partper million to 10,000 parts per million.
 6. The method of claim 1,wherein the organic additive is selected from the group consisting ofthioureas, thiazines, sulfonic acids, sulfonic acids, allyl phenylsulfone, sulfamides, dithioxo-bishydroxylaminomolybdenum complex, andderivatives thereof.
 7. The method of claim 1, wherein the solutioncomprises an alkane sulfonic acid selected from the group consisting ofmethane sulfonic acid, ethane sulfonic acid, propane sulfonic acid, andbutane sulfonic acid, and wherein the alkane sulfonic acid is at aconcentration of 0.1 M to 2 M.
 8. The method of claim 1, wherein thep-type semiconductor has a ratio of Cu/(In+Ga) at 0.8 to 0.9 and a ratioof Ga/(Ga+In) at 0.3 to 0.33.
 9. A method for forming a thermalinterface, the method comprising: electroplating a layer of gallium or agallium alloy onto a heat emitting surface coupled to a microprocessor,wherein electroplating the gallium or gallium alloy comprises contactingthe heat emitting surface with a plating bath free of complexing agentscomprising a gallium salt and an optional organic additive comprising atleast one sulfur atom, and a solvent; adjusting a pH of the plating bathto a range selected from the group of a pH of greater than about zero toless than 2.6 and a pH greater than about 12.6 to about 14, and applyinga current to electroplate the heat emitting surface to produce a layerof the gallium or gallium alloy; and coupling a heat sink or a heatspreader to the layer of gallium or the gallium alloy to form thethermal interface.
 10. The method of claim 9, wherein the plating bathcomprises an alkane sulfonic acid selected from the group consisting ofmethane sulfonic acid, ethane sulfonic acid, propane sulfonic acid, andbutane sulfonic acid, and wherein the alkane sulfonic acid is at aconcentration of 0.1 M to 2 M.
 11. The method of claim 9, wherein thesolution comprises sodium sulfate at a concentration of 0.01 M to 2 M.12. The method of claim 9, wherein the organic additive is selected fromthe group consisting of thioureas, thiazines, sulfonic acids, sulfonicacids, allyl phenyl sulfone, sulfamides,dithioxo-bishydroxylaminomolybdenum complex, and derivatives thereof.